1. Field of the Invention
The present invention relates to a semiconductor laser structure and, more particularly, to a semiconductor laser array which includes a current confinement structure and a bult-in refractive index difference, the refractive index being the vaule of the real part only.
2. Description of the Prior Art
A semiconductor laser has widely been used as a light source in an optical information processing system such as an optical disc system and a laser printer. Rapid processing is required in such an optical information processing system as the amount of information to be handled increases. To ensure rapid processing, the semiconductor laser must emit the laser beam at a high power level in a stable operating range. However, in the conventional semiconductor laser having a single active region, the practical maximum output is around 50 mW.
To enhance the output level, a laser device has been proposed, wherein a plurality of semiconductor lasers are aligned in a parallel fashion, and the plurality of semiconductor lasers are optically, phase coupled to each other so as to emit the laser beam in a single phase. This is referred to as the phase coupled laser array. However, in a semiconductor laser array of the gain guide type, the gain is substantially reduced at the coupling region positioned between the adjacent two laser emitting regions and, therefore, the electric field has the phase difference of 180 degrees at the adjacent two laser emitting regions. The far field pattern has a plurality of peaks as shown in FIG. 1. Thus, the semiconductor laser array of the gain guide type can not endure for practical use.
To improve the above-mentioned problems, a semiconductor laser array of the index guide type has been proposed. For example, D. E. Ackley et al of Hewlett-Packard Laboratories proposed a semiconductor laser array of the leaky mode built-in type (Appl. Phys. Letters, 39(1), July 1, 1981. P27). The proposed laser array ensures an effective coupling of the laser emitting regions, but has two peaks in the far field pattern because of the leaky mode.
D. Botez et al of RCA Laboratories proposed a CSP-LOC (Channeled-Substrate-Large-Optical-Cavity) laser (document of IOOC, 1983. 29B5-2). The proposed semiconductor laser utilizes the distribution of an effective refractive index which is formed by the coupling to the GaAs substrate. The region disposed between the adjacent two laser emitting regions has a high absorption coefficient. The refractive index difference is not obtained when the absorption coefficient is minimized. Accordingly, it is difficult to reduce the phase difference between the adjacent two laser emitting regions to zero.
D. E. Ackley et al of Hewlett-Packard Laboratories further proposed the semiconductor laser array of the ridge-type (Appl. Phys. Letters, 42(2), 15 January 1983. P152). Each pair of the adjacent two laser emitting regions has a phase difference of 180 degrees because of the high absorption caused by the electrode disposed at the coupling region of the adjacent two laser emitting regions.